Air may be present in dissolved and in undissolved forms in pressure fluids such as hydraulic fluids. Although dissolved air is not visible in the fluid or oil, it is always present to a certain extent. Undissolved air is not always present in the fluid or oil, but when it is, it is visible as a second phase. Although the properties of pressure fluids, such as viscosity, compression modulus, and lubricating capacity, are substantially impaired by undissolved air, dissolved air also affects certain properties of the pressure fluid, for example ageing behavior and cavitation tendency.
The maximum amount of air that can be dissolved in the fluid concerned is determined by the saturation curve of the fluid concerned. However, in principle, solubility increases with increasing pressure and to a lesser extent is also dependent on temperature. Because pressures and temperatures in hydraulic systems vary over time, as well as according to location, a drop in pressure and consequent lowering of the solubility limit can result in air dissolving out during operation. As a result, the formation of a second phase (bubbles), and thus damage, for example by flow cavitation, can be expected, especially in areas of low static pressure, such as control cross sections of valves. Because the outgassing rate is greater than the dissolution rate, resulting air bubbles remain even when the pressure in the fluid increases again, thereby altering the properties of the pressure fluid on the one hand and possibly resulting in damages due to cavitation erosion upon a subsequent pressure increase on the other hand. In order to ensure reliable operation of hydraulic systems, it is necessary to take measures for degasifying the pressure fluid. Vacuum evaporation is the process currently employed for achieving a particularly effective degasification, in which the degassed fluid contains only a small residual contamination. Although this has the advantage that water is simultaneously eliminated, the very high energy requirement and the elaborate construction of the apparatus are disadvantageous. In addition there is usually an undesired heating of the fluid. Also, it is not possible to integrate the degasifying apparatus in a main fluid flow of the fluid system.